Production of saturated aliphatic nitriles from aldehydes and imines prepared therefrom



United States Patent v 2,755,300 PRODUCTION OF SATURATED ALIPHATIC NI-TRILES FROM ALDEHYDES AND IMINES PRE- PARED THEREFROM No Drawing.Application April '23, 1952',

Serial No. 283,971

4 Claims. (Cl. 260-4651) The present invention relates to thepreparation of saturated; aliphatic nitriles and to the synthesis ofintermediates useful in the production thereof. More specifically theinvention is concerned with the conversion into saturated aliphaticnitriles of saturated aliphatic aldehydes, their aldol condensationproducts, unsaturated aldehydes prepared from the latter, otherunsaturated aldehydes, and unsaturated aldimines prepared vfrom any ofsaid aldehydes and said condensation products. In one of its mostimportant embodiments the invention relates to the synthesis fromstraight chain saturated aliphatic aldehydes or unsaturated aliphaticaldehydes preparedby aldol condensation and dehydration of said straightchain saturated aliphatic aldehydes, of branched chain (ix-substituted)saturated aliphatic nitriles containing twicev thenumber of carbon atomsas contained in said saturated aldehydes.

The production of unsaturated nitriles from unsaturated orsaturatedaldehydes is the subject of U. S. Patent No. 2,412,437. Mycopending U. S. Patent application, Serial No. 195,899, filed November15, 1950, now abandoned,

relates to the preparation of aliphatic nitriles particularly saturatedaliphatic nitriles from aliphatic'aldehydes and ammonia,employingsilver-zinc catalysts. 7

From the above and other art it-is evident thatfone method for-preparinga relatively highera-substituted saturated nitrile from a relativelylower unsubstituted (i. e. straight chain) saturated aldehyde resides inthe following steps: i I i 1:; Saturated 1ower'a1dehyde'(A) aldoleondensationaldol '2. Aldol -Hz'o unsaturated higheraldehydMB) 3.Unsaturated higher aldehyde (B) +H2 saturated higher alde- .hyde (B)corresponding saturated nitrile +13 I In accordance withthe four-step"process suggested above,- propionaldehyde for instance would beconverted 4. Saturated higher aldehyde (B) +NH to '2-methylpentenal,which then would .be'hydrogenated L.

.' process.

It is obvious that it would be quite advantageous to eliminate the stepof hydrogenation (step 3) if adequate results could be obtained'withoutit, since hydrogenation is a-co'stly procedure in commercial practice.It also is obvious that a reduction in the number of steps required toconvert'the saturated lower aldehyde to the higher tat-substitutedsaturated nitrile would be beneficial as a practical matter.

I have found that the prior four-step method may be accomplished in twosimplified operations whereby the costly step of hydrogenation may beobviated. This is achieved by forming from the saturated lower aldehydesin a single liquid phase conversion unsaturated imines of the higherunsaturated u-substituted aldehyde. In an abbreviated second step theunsaturated branched chain imine, through reaction with ammonia in vaporphase over proper catalysts, is both hydrogenated to a saturated imineand dehydrogenated to a nitrile. This is possible sinceit is feasible toemploy for the production of nitriles from iminesand ammonia catalystswhich are hydrogenation as well as dehydrogenation catalysts. Thus, ineffect the unsaturated imine serves as its own hydrogen acceptor.

Accordingly an important object of the present invention is to provide anew process by which the costly hydrogenation step may be avoided. Asecond important object concerns a method of carrying out the aldolcondensation, dehydration and imine formation as a single step. Thus itis an object .of the invention to provide :a new process for thepreparation of saturated aliphatic nitriles, more specifically a processwhereby saturated aliphatic nitriles may be prepared in a two-stepprocedure from lower saturated aldehydes; Another object is to preparebranched chain (or-substituted) aliphatic nitriles. A'further object is,-the preparation of branched chain (asubstituted) aliphatic-nitrilesfrom saturated straight chain aldehydes. Still another object is thepreparation of saturated aliphaticnitriles containing twice the numberof carbon atoms as the saturated aliphatic aldehyde starting materialused for their preparation. A still further object is the provision of amethod for preparing saturated aliphatic nitriles from unsaturatedaldehydes of the same number of carbon atoms, and the provision of amethod for the preparation of such nitriles from imines, specificallyunsaturated imines. Another object concerns the preparation ofunsaturated imines from saturated aliphatic aldehydes of fewer carbonatoms. Another object relates to, the preparation of unsaturatedbranched chain (cc-Substituted) normally-liquid imines useful for thepreparation prepared from theproducts, of an aldol condensation and thesynthesis of aliphatic nitriles by the vapor phase catalytic reaction ofammonia with products of aldol condensation ofaliphatic aldehydes andtheir imines. Another object relates to the preparation ofu-methylvaleronitrile and similar saturated lower aliphatic nitrileshaving an ix-substituted s'ubstituent.

Still other objects of the invention are the elimination of theexpensive purification steps and avoidance of use of strong basecatalysts for the aldol condensation, both of which are necessary to theabove-described four step Astill further object of the invention is toprovide a new aldimine'compound having the empirical formula C9H15Nwhich upon hydrolysis reverts to one molecule of propionaldehyde and.one molecule of Z-methylpentenal- 'dehyde. This compound is believed tohave the structure:

Pitented July 17,

Additional objects will be evident from the following specification andclaims.

In accordance with my invention saturated straight chain aliphaticaldehydes of three or more carbon atoms are employed to preparesaturated branched chain (01'- substituted) higher aliphatic nitrilescontaining a main chain of six or more carbon atoms. In general it maybe said that the process comprises the steps of:

l. Reacting a saturated aliphatic aldehyde with aqueous ammonia andrecovering from the reaction products an unsaturated branched chainaldehyde imine containing a multiple of the number of carbon atoms ofthe aldehyde starting material.

2. Passing the unsaturated imine with ammonia over a catalyst at anelevated temperature to thereby produce a branched chain saturatedaliphatic nitrile containing twice the number of carbon atoms as thestarting aldehyde.

This represents the preferred embodiment in terms of which the inventionwill be explained. However, various modifications which do not fallwithin the definition given above will be apparent in the presentdisclosure and are not to be excluded from the inventive concept. One ofthese modifications, for instance, is that wherein a starting mixture ofacetaldehyde and propionaldehyde is employed to prepare a five carbonatom nitrile through the intermediate imine of tiglic aldehyde (i. e.dimethyl acrolein). Thus, a generic process in accordance with theinvention provides for the preparation of nitriles corresponding to theimines of a,/3-alkyl-disubstituted acroleins, that is saturatedaliphatic branched chain (at-substituted) nitriles of five or morecarbon atoms.

cirt' oaeorri ti-cnr CH: CHI

It is possible that hydrogen is never actually split out of the nitrilein the reaction 2(a) and ad'dedagain in reaction 2(b) but that insteadrearrangement takes place to form the saturated nitrile. p

In consonance with the main inventive concept outlined above, otherembodiments of the invention comprise the step of converting asalternative starting materials, intermediates of the above process, i.e. unsaturated branched chain (at-substituted) aldehydes and iminesthereof to saturated branched chain (at-substituted) triles of the samenumber of carbonatoms by means of imine hydrogenation anddehydrogenation carried out in the vapor phase. 7 p

It will be apparent that the process is useful prirna'rily for theproduction of saturated aliphatic branched chain (ct-substituted)nitriles having a main chain of five 'or more carbon atoms, e. g.a-methylvaleronitrile, a-ethylcapronitrile, etc. However, a number ofadditional products result as byproducts of the reaction, generally inminoramounts, Where the initial starting material is propionaldehyde forexample, substantial portions of 2- methyl Z-pentenonitrile will berecovered along with propionitrile and unconverted imines.

Other aliphatic alkyl nitriles which may be prepared appropriately ,bymeans of the instant process include u-propylenanthonitrile,a-butylcaprylonitrile, a-methylbutyronitrile, a-methylcapronitrile,a-ethylvaleronitrile, etc.

Where the teachings of the presents invention are utilized for thepreparation of saturated aliphatic nitriles by a two-step procedurestarting with saturated unsubstituted sraight chain aliphatic aldehydesand going through an intermediate phase of aldol condensation andformation of unsaturated aliphatic aldimines, it is obvious that thenitriles produced will always have an even number of carbon atoms, whichnumber is twice that of the lower aldehyde starting material except whena mixture of lower aldehydes is used as starting material. In the latterevent, some odd number carbon atom product may result. It further isapparent that the nitriles will have a branched chain if the aldehydestarting material is propionaldehyde or a higher aliphatic straightchain aldehyde. The lower aldehydes, i. e. formaldehyde and acetaldehydemay be said to be generally inoperative since formaldehyde does notundergo the usual aldol condensation and dehydration, and acetaldehydemay undergo conversion to crotonaldehyde which latter usually behavespeculiarly in reaction with ammonia as further discussed below. However,it is likely that acetaldehyde may function within the terms of theinvention to produce some of the unsaturated higher imine under optimumconditions.

In other embodiments of the invention intermediate aldehydes which neednot pass through aldol formation and dehydration may be employed asstarting material. Thus the chosen aldehyde starting material may be anunsaturated branched chain (substituted at the a carbon) aliphaticaldehyde which is converted to the corresponding unsaturated imine whichis in turn hydrogenated and dehydrogenated to the correspondingsaturated branched chain aliphatic nitrile. In this case it is clearthat the resulting nitrile in accordance with the invention will havethe same number (rather than twice the number) of carbon atoms as thealdehyde starting material and this number of course is not necessarilyan even number. By the same token, the resulting nitrile will have achain identical with that of the starting material. Thus, the inventiveconcept comprising this embodiment resides in the liquid phasepreparation of an unsaturated aldimine of the unsaturated startingmaterial and subsequent hydro genation-dehydrogenation of the imine toform a nitrile corresponding to the aldehyde starting material buthaving a parafiinic linkage where the original aldehyde or imine has anolefinic linkage. By means of this embodiment, saturated nitriles ofrelatively fewer carbon atoms and/ or having odd numbers of carbon atomsmay be prepared. For instance, valeronitrile may be prepared from2-penten-l-al. However, as indicated above, the lower unsaturatedaldehydes such as acrolein and crotonaldehyde react with aqueous ammoniato form very high molecular weight condensation products rather than theimines containing only three or less alkyl radicals. How ever, undercertain circumstances it may be possible to react acrolein orcrotonaldehyde to produce nitriles provided they are vaporized andheated close to reaction temperature before mixing with ammonia andpassing over the catalyst.

The phase of the invention which resides in aldimine preparation, thatis, preparation of an unsaturated branched chain (at-substituted)aliphatic aldimine from an aliphatic aldehyde is somewhat more flexiblein that it only necessitates use ofan aliphatic aldehyde which willundergo an aldol condensation and subsequent dehydration to produce animine relatively more unsaturated and having a relatively more-branchedchain, the

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imine being of an even number of carbon atoms which is a multiple of thenumber of carbon atoms contained in the starting aldehyde, except forthe possibilities when mixed aldehydes are used as starting materials.

The higher nitriles which 'canbe prepared by the process of theinvention are ,useful as organic intermediates and certain of them maybeemployed as modifiers or solvents for certain commercially importantnitrile polymers. j

. As indicated above, it may be possible to react-acetaldehyde withaqueous ammonia and vaporize the product with excess ammonia overa'catalyst and thereby produce n-butyronitrile and acetonitriles.-However, this reaction involving a short chain saturated aldehyde isnot considered to be Within 'the more preferred embodiment of theinvention as discussed in connection with saturated aldehydes of threeor more carbon atoms.

As indicated above it is in keeping with the concept of this inventionto practice nitrile preparation starting with a normally liquid imine,particularly an unsaturated imine, preferably of a branched chain(or-substituted) and utilizing step 2 only of the process. Where thisembodiment is employed suitable imine starting material, depending uponthe particular nitrile which it is desired to produce, includes iminessuch as:

In step 2 of the process, e. g. conversion of the unsaturated imine tothe saturated nitrile, the ammonia should be employed in excess of thetheoretical amount required. As will be noted from the examples givenbelow the optimum mole ratio of ammonia to imine is in the range ofabout 2 to 1 to 4 to 1.

The hydrogenation-dehydrogenation of the imine is accomplished bycontacting a preferably preheated gaseous mixture of the ammonia andimine with an appropriate hydrogenation-dehydrogenation catalystmaintained at a temperature in the range of about 250 C. to 500 C.preferably about 325 C. to 400 .C. for a period in the range of about0.01 to 2.0 seconds at an appropriate space velocity, e. g. 300 or 350.

The ammonia and imine may be mixed before reaching or within thecatalyst bed, any of a number of suitable procedures standard to thecatalytic vapor phase preparation of nitriles being appropriate.Diluents may be employed where desirable in accordance with effectiveprocedures well-known in the art. Diluents may be aptly utilized tosupply some of the heat required for the reaction and/ or to facilitatecontrol of temperature in the reaction zone (catalyst bed).

The aldol condensation phase of the imine formation takes place in thepresence of aqueous ammonia with suitably an equimolar proportion ofreactants. The aqueous ammonia advantageously may be of a concentrationwithin the range of 1 to 30%, although the concentration of ammonia isnot critical as long as an excess is present. Normally no extraneouscatalyst is required and the ammonia serves also to supply the necessaryreactant for conversion of the free aldehyde group to form the imine.Imine formation apparently takes place through an intermediatealdehyde-ammonia which is dehydrated in the course of the reaction, thereaction mixture being maintained at a temperature suitably in the rangeof 30-80" C. The imine hydrogen may condense with another carbonyl groupto form N-substituted polyalkyl imines or alkenyl imines together withsimple aldimines which separate as a water insoluble layer.

Any number of suitable hydrogenation-dehydrogenation catalysts may beemployed for conversion of the imine to nitrile. However it has beendetermined that two metal catalysts, to wit copper-zinc and silver-zinc,are particularly advantageous. These catalysts and methods for theirpreparation are per se known to the art. See for instance, myaforementioned application.

I Thirty moles of propionaldehyde were fed into moles of 10% ammoniumhydroxide with cooling at 20 C. The/solution was then heated to 75" C.for 1 hour. An insoluble imine layer equal in volume to per cent of the.propionaldehyde separated. The aqueous layer was refortified withammonia and used-toreact with more propionaldehyde.v Alternately, itcould be extracted to recover more imine. The crude imine productcontained 1 nitrogen for each 4.5 carbon atoms. Distillationseparatedammonia, water, propionaldirnine, and higher imines. Over 40per cent of the product was a cut, B. P. 98/50 mm., which upon refluxing"'with' aqueous hydrochloric acid hydrolyzed to give equal moles ofpropionaldehyde and 2-methylpentenal. Higher cuts also hydrolyzed toproduce additional Z-methylpentenal.

Example 2 The crude imine product separated in Example 1 was fed withexcess ammonia over a copper-zinc catalyst at 350 C. and 300 spacevelocity. The products were condensed and distilled to producepropionitrile, Z-methylpentenonitrile, Z-methylpentanonitrile, B. P. C./735,

and unreacted imines. Analysis showed the C6 nitrile to be mainly thesaturated 2-methylpentanonitrile. Some Z-methylpentenonitrile was alsoproduced. If desired the unsaturated nitrile and unreacted imines couldbe recycled.

Example 3 Normal butyraldehyde was fed into an equal molar amount ofammonium hydroxide at 2030 C, and then warmed to reflux. The mixture ofimines of butyraldehyde and 2-ethylhexenaldehyde separated as a toplayer. These were decanted and then vaporized in a stream of ammoniagas, and passed over a catalyst composed of 54 per cent silver and 46per cent zinc. The catalyst was heated to 400 C. A space velocity of 350was used. Distillation separated normal butyronitrile,2-ethylhexenonitrile and Z-ethylhexanonitrile. Very little substitutedpyridines were produced.

Example 4 98 gms. of Z-methylpentenal was reacted with excess aqueousammonia to produce mixed imines at a 20-80 C. reaction temperature. Themixed imine layer was separated and fed with 70 gms. of ammonia over acopper-zinc catalyst at 325 C. The products were condensed and theorganic layer fractionated to isolate 55 gms. of 2-methypentanonitrile(u-methylvaleronitrile).

Example 5 Normal butyraldehyde was condensed with alkaline catalyst toprepare 2-ethylhexenal. The 2-ethylhexenal was reacted with excessaqueous ammonia to produce mixed imines. These crude imines along with a3-molar excess of ammonia were passed over a silver-zinc catalyst at 375C. The products of reaction were condensed. The organic layer wasseparated and washed free of unreacted imines and amines with diluteactid and :then azeotropically distilled. 2-ethylhexanonitrile wasobtained in good yield. Analysis for unsaturation showed that verylittle 2-ethylhexenonitrile is present.

The vapor phase conversion to nitrile takes place at atmosphericpressure, but the reaction suitably may be conducted at increased ordecreased pressures and the nitrile products may be separated, recoveredand purified in accordance with standard procedures.

The production of nitriles from aldehyde imines is disclosed and claimedin my copending U. S. patent application Serial No. 266,779, filedJanuary 16, 1952, of which the present application is a continuation inpart. The prior application also discloses and claims certain imines andmethods for the preparation thereof.

I claim:

1. Method for the preparation of a saturated aliphatic nitrilecomprising the steps of vaporizing an unsaturated aliphatic primaryimine, reacting the thus vaporized imine with an amount of gaseousmmonia in excess of the ratio of one mole of ammonia per mole of theimine, over a dehydrogenadon-hydrogenation catalyst selected from thegroup consisting of copper-zinc and silver-zinc at a temperature in therange of about 250 C. to about 500 C., and recovering saturated nitrilethus produced.

2. Method as defined in claim 1 wherein the primary imine is an alkenylimine.

3. Method as defined in claim 2 wherein the alkenyl imine is a branchedchain imine.

8 4. Method as defined in claim 3 wherein the branched chain imine hasthe formula CHI References Cited in the file of this patent UNITEDSTATES PATENTS OTHER REFERENCES Mowry: Chemical Reviews, vol. 42, pp.249-50 (1948).

1. METHOD FOR THE PREPARATION OF A SATURATED ALIPHATIC NITRILECOMPRISING THE STEPS OF VAPORIZING AN UNSATUREATED ALIPHATIC PRIMARYIMINE, REACTING THE THUS VAPORIZED IMINE WITH AN AMOUNT OF GASEOUSMMONIA IN EXCESS OF THE RATIO OF ONE MOLE OF AMMONIA PER MOLE OF THEIMINE, OVER A DEHYDROGENATION-HYDROGENATION CATALYST SELECTED FROM THEGROUP CONSISTING OF COPPER-ZINC AND SILVER-ZINC AT A TEMPERATURE IN THERANGE OF ABOUT 250* C. TO ABOUT 500* C., AND RECOVERING SATURATEDNITRILE THUS PRODUCED.